All U.S. patents cited below are herein entirely incorporated by reference.
As used herein, the term “thermoplastic” is intended to mean a polymeric material that will melt upon exposure to sufficient heat but will retain its solidified state, but not prior shape without use of a mold or like article, upon sufficient cooling. Specifically, as well, such a term is intended solely to encompass polymers meeting such a broad definition that also exhibit either crystalline or semi-crystalline morphology upon cooling after melt-formation. Particular types of polymers contemplated within such a definition include, without limitation, polyolefins (such as polyethylene, polypropylene, polybutylene, and any combination thereof), polyamides (such as nylon), polyurethanes, polyesters (such as polyethylene terephthalate), and the like (as well as any combinations thereof).
Thermoplastics have been utilized in a variety of end-use applications, including storage containers, medical devices, food packages, plastic tubes and pipes, shelving units, and the like. Such base compositions, however, must exhibit certain physical characteristics in order to permit widespread use. Specifically within polyolefins, for example, uniformity in arrangement of crystals upon crystallization is a necessity to provide an effective, durable, and versatile polyolefin article. In order to achieve such desirable physical properties, it has been known that certain compounds and compositions provide nucleation sites for polyolefin crystal growth during molding or fabrication. Generally, compositions containing such nucleating compounds crystallize at a much faster rate than unnucleated polyolefin. Such crystallization at higher temperatures results in reduced fabrication cycle times and a variety of improvements in physical properties, such as, as one example, stiffness.
Such compounds and compositions that provide faster and or higher polymer crystallization temperatures are thus popularly known as nucleators. Such compounds are, as their name suggests, utilized to provide nucleation sites for crystal growth during cooling of a thermoplastic molten formulation. Generally, the presence of such nucleation sites results in a larger number of smaller crystals. As a result of the smaller crystals formed therein, clarification of the target thermoplastic may also be achieved, although excellent clarity is not always a result. The more uniform, and preferably smaller, the crystal size, the less light is scattered. In such a manner, the clarity of the thermoplastic article itself can be improved. Thus, thermoplastic nucleator compounds are very important to the thermoplastic industry in order to provide enhanced clarity, physical properties and/or faster processing.
As an example of one type of nucleator, dibenzylidene sorbitol derivative compounds are typical nucleator compounds, particularly for polypropylene end-products. Compounds such as 1,3-O-2,4-bis(3,4-dimethylbenzylidene) sorbitol, available from Milliken Chemical under the trade name Millad® 3988 (hereinafter referred to as 3,4-DMDBS), provide excellent nucleation characteristics for target polypropylenes and other polyolefins. Other well known compounds include sodium benzoate, sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl) phosphate (from Asahi Denka Kogyo K.K., known as and hereinafter referred to as NA-11), aluminum bis[2,2′-methylene-bis-(4,6-di-tert-butylphenyl)phosphate] (also from Asahi Denka Kogyo K.K., which is understood to be known as and hereinafter referred to as NA-21), talc, and the like. Such compounds all impart high polyolefin crystallization temperatures; however, each also exhibits its own drawback for large-scale industrial applications.
Other acetals of sorbitol and xylitol are typical nucleators for polyolefins and other thermoplastics as well. Dibenzylidene sorbitol (DBS) was first disclosed in U.S. Pat. No. 4,016,118 by Hamada, et al. as effective nucleating and clarifying agents for polyolefin. Since then, large numbers of acetals of sorbitol and xylitol have been disclosed, including bis(p-methylbenzylidene) sorbitol (hereinafter referred to as 4-MDBS). Representative references of such other compounds include Mahaffey, Jr., U.S. Pat. No. 4,371,645 [di-acetals of sorbitol having at least one chlorine or bromine substituent].
As noted above, another example of the effective nucleating agents are the metal salts of organic acids. Wijga in U.S. Pat. Nos. 3,207,735, 3,207,736, and 3,207,738, and Wales in U.S. Pat. Nos. 3,207,737 and 3,207,739, suggest that aliphatic, cycloaliphatic, and aromatic carboxylic, dicarboxylic or higher polycarboxylic acids, and corresponding anhydrides and metal salts, are effective nucleating agents for polyolefin. They further state that benzoic acid type compounds, in particular sodium benzoate, are the best nucleating agents for their target polyolefins.
Another class of nucleating agents was suggested by Nakahara, et al. in U.S. Pat. No. 4,463,113, in which cyclic bis-phenol phosphates was disclosed as nucleating and clarifying agents for polyolefin resins, as well as U.S. Pat. No. 5,342,868 to Kimura, et al. Compounds that are based upon these technologies are marketed under the trade names NA-11 and NA-21, discussed above.
Furthermore, a certain class of bicyclic compounds, such as bicyclic dicarboxylic acid and salts, have been taught as polyolefin nucleating agents as well within Patent Cooperation Treaty Application WO 98/29494, 98/29495 and 98/29496, all assigned to Minnesota Mining and Manufacturing. The best working examples of this technology are embodied in disodium bicyclo[2.2.1]heptene dicarboxylate and camphanic acid.
The efficacy of nucleating agents is typically measured by the peak crystallization temperature of the polymer compositions containing such nucleating agents. A high polymer peak crystallization is indicative of high nucleation efficacy, which generally translates into fast processing cycle time and more desirable physical properties, such as stiffness/impact balance, etc., for the fabricated parts. Compounds mentioned above all impart relatively high polyolefin crystallization temperatures; however, each also exhibits its own drawback for large-scale industrial applications.
For example, it is very desirable that the effective nucleating compounds exhibit a very high peak crystallization temperature, for example, above 125° C. within a test homopolymer polypropylene that, when unnucleated exhibits a number of different characteristics such as a density of about 0.9 g/cc, a melt flow of about 12 g/10 min, a Rockwell Hardness (R scale) of about 90, a tensile strength of about 4,931 psi, an elongation at yield of about 10%, a flexural modulus of about 203 ksi, an Izod impact strength of about 0.67 ft-lb/in, and a deflection temperature at 0.46 mPa of about 93° (which provides a homopolymer exhibiting an isotacticity of between about 96 and 99%), wherein said peak crystallization temperature is measured by differential scanning calorimetry in accordance with ASTM Test Method D3417-99 modified to measure at heating and cooling rates of 20° C./minute. Such a polypropylene homopolymer provides an effective test subject for this purpose due to the general uniformity of product available (and thus better uniformity in peak crystallization temperature, etc., results, therein for samples of such a thermoplastic), as well as the widespread use of such a thermoplastic. Of course, it should be well understood by the ordinarily skilled artisan that such a test homopolymer is not the only thermoplastic in which the inventive nucleating agent may be present; it is solely a test formulation in order to determine the highest peak crystallization temperature, etc., for certain inventive nucleating agents under certain conditions. Of the nucleating agents mentioned above, only camphanic acid exhibits such a high polymer peak crystallization temperature within such a test homopolymer propylene formulation. However, as shown in the comparative examples within this invention, camphanic acid exhibits very poor thermal stability, where it tends to vaporize and accumulate on the surface of plastic processing equipments during processing. This phenomenon is generally referred to as “plate out” within the plastics industry. The “plate out” effect of this additive make it impractical for any commercial use. Thus, the combination of very high polymer peak crystallization temperature (thus highly efficient nucleation) and a low degree of fugitivity (and thus high thermal stability and low plate-out characteristics) within the target polymers (e.g., preferably polyolefins such as polypropylene) is very desirable within the plastics industry, particularly where the peak crystallization temperature is measured above 126° C. within a homopolymer polypropylene measured by differential scanning calorimetry at a rate of 20° C./minute. So far, such a combination has not been provided within this intensively studied area of polymer nucleating agents.
Beyond high polymer crystallization temperature and low fugitivity, there are a number of other performance characteristics important for the practical use of such nucleating agents. For example, one of great interest is the compatibility of such compounds with different additives widely used within typical polyolefin (e.g., polypropylene, polyethylene, and the like) plastic articles. As noted previously, calcium stearate compatibility is particularly important. Unfortunately, most of the nucleator compounds noted above (such as sodium benzoate, NA-11, disodium bicyclo[2.2.1]heptene dicarboxylate) exhibit deleterious nucleating efficacy when present with such compounds within polyolefin articles. It is generally speculated that the calcium ion from the stearate transfers positions with the sodium ions of the nucleating agents, rendering the nucleating agents ineffective for their intended function. As a result, such compounds sometimes exhibit unwanted plate-out characteristics and overall reduced nucleation performance as measured, for example, by a decrease in crystallization temperature during and after polyolefin processing of greater than 2° C. as compared to the peak crystallization temperature of the nucleated polymer with no calcium stearate present therein. In order to avoid combinations of these standard nucleators and calcium salts, other nonionic acid neutralizers, such as dihydrotalcite (DHT4-A), would be necessary for use in conjunction with such nucleators. Such a combination, however, has proven problematic in certain circumstances due to worsened aesthetic characteristics (e.g., higher haze), and certainly higher costs in comparison with standard calcium salts.
Other problems encountered with the standard nucleators noted above include inconsistent nucleation due to dispersion problems, resulting in stiffness and impact variation in the polyolefin article. Substantial uniformity in polyolefin production is highly desirable because it results in relatively uniform finished polyolefin articles. If the resultant article does not contain a well dispersed nucleating agent, the entire article itself may suffer from a lack of rigidity and low impact strength.
Furthermore, storage stability of nucleator compounds and compositions is another potential problem with thermoplastic nucleators and thus is of enormous importance. Since nucleator compounds are generally provided in powder or granular form to the polyolefin manufacturer, and since uniform small particles of nucleating agents are imperative to provide the requisite uniform dispersion and performance, such compounds must remain as small particles through storage. Certain nucleators, such as sodium benzoate, exhibit high degrees of hygroscopicity such that the powders made therefrom hydrate easily resulting in particulate agglomeration. Such agglomerated particles may require further milling or other processing for deagglomeration in order to achieve the desired uniform dispersion within the target thermoplastic. Furthermore, such unwanted agglomeration due to hydration may also cause feeding and/or handling problems for the user.
Some nucleating agents, such as certain DBS derivatives, exhibit certain practical deficiencies such as a tendency to plate-out at high processing temperatures. DBS derivatives, particularly where the aromatic rings are mono-substituted, show much improved thermal stability. However, such compounds also tend to exhibit undesirable migratory properties coupled with problematic organoleptic deficiencies within certain polyolefin articles. As a result, such compounds cannot be widely utilized in some important areas, such as within medical devices (e.g., syringes, and the like) and food packaging.
These noticeable problems have thus created a long-felt need in the plastics industry to provide such compounds that do not exhibit the aforementioned problems and provide excellent peak crystallization temperatures and low fugitivity for the target polyolefins themselves. To date, the best compounds for this purpose remain those noted above. To date, nucleators exhibiting exceptionally high peak crystallization temperatures, low fugitivity, low hygroscopicity, excellent thermal stability, and non-migratory properties within certain target polyolefins, and compatibility with most standard polyolefin additives (such as, most importantly, calcium stearate) have not been available to the plastics industry.